1. [Field of the Invention]
The present invention relates to an apparatus for recording images on a recording sheet, such as paper, by allowing recording particles to jump and directly attach to the recording sheet.
2. [Description of the Related Art]
U.S. Pat. No. 5,477,250, issued on Dec. 19, 1995, discloses a recording apparatus. This recording apparatus comprises a cylindrical toner retaining member, which is rotatable, for retaining charged toner particles on its outer peripheral surface, and a back electrode spaced from the toner retaining member. The back electrode is electrically connected to a power source so that an electric field is formed to attract the charged toner particles on the toner retaining member toward the back electrode. An insulating plate provided with a plurality of apertures, through which the toner particles can pass, is disposed between the toner retaining member and the back electrode. In addition, the insulating plate is provided with a ring-shaped electrode surrounding each of the apertures.
In the above-mentioned recording apparatus, when a signal corresponding to image data is applied to the electrode, the toner particles existing at a position on the toner retaining member, where is opposed to the electrode, jump into the corresponding aperture. After passing through this aperture, the toner particles attach to a recording sheet, and an image corresponding to the image data is recorded on the recording sheet.
In the above-mentioned recording apparatus, the toner particles disperse while jumping, and attach to the recording sheet, thereby forming a dot that is blurred in outline and has a low density. A recorded image obtained in this way is therefore blurred in outline and lacks sharpness.
In order to solve this problem, a method is available wherein a guard electrode surrounding the aperture is provided on the back electrode side of the insulating plate with respect to the ring-shaped electrode, and a voltage having the same polarity as that of the charged toner particles is applied to this guard electrode so that the group of the toner particles jumping inside the aperture is biased by electric repulsion force to converge inwardly in the radial direction. With this method, a dot that is sharp in outline and has a high density can be formed, and an image having high definition can be obtained.
In the case of the above-mentioned recording apparatus without the guard electrode, the density of an image to be recorded is apt to rise as the application time of the signal to be applied to the ring-shaped electrode is prolonged. Therefore, in the case of carrying out gradation representation by using this recording apparatus, the application time of the signal to be applied to the ring-shaped electrode is generally prolonged to raise the image density.
However, in the recording apparatus with the guard electrode, it was found that the image density rises once as the application time of the signal to be applied to the ring-shaped electrode is prolonged, but lowers after the application time goes beyond a certain time, depending on the voltage potential applied to the guard electrode, in some cases. In these cases, it is impossible to perform gradation control by using the method of simply prolonging the application time of the signal to be applied to the ring-shaped electrode to raise the image density.